Re: [PATCH v15.1 04/13] btrfs: dedupe: Introduce function to remove hash from in-memory tree

[Josef Bacik <josef@toxicpanda.com>]

On Tue, Nov 06, 2018 at 02:41:13PM +0800, Lu Fengqi wrote:
> From: Wang Xiaoguang <wangxg.fnst@cn.fujitsu.com>
> 
> Introduce static function inmem_del() to remove hash from in-memory
> dedupe tree.
> And implement btrfs_dedupe_del() and btrfs_dedup_disable() interfaces.
> 
> Also for btrfs_dedupe_disable(), add new functions to wait existing
> writer and block incoming writers to eliminate all possible race.
> 
> Cc: Mark Fasheh <mfasheh@suse.de>
> Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
> Signed-off-by: Wang Xiaoguang <wangxg.fnst@cn.fujitsu.com>
> Signed-off-by: Lu Fengqi <lufq.fnst@cn.fujitsu.com>
> ---
>  fs/btrfs/dedupe.c | 131 +++++++++++++++++++++++++++++++++++++++++++---
>  1 file changed, 125 insertions(+), 6 deletions(-)
> 
> diff --git a/fs/btrfs/dedupe.c b/fs/btrfs/dedupe.c
> index 784bb3a8a5ab..951fefd19fde 100644
> --- a/fs/btrfs/dedupe.c
> +++ b/fs/btrfs/dedupe.c
> @@ -170,12 +170,6 @@ int btrfs_dedupe_enable(struct btrfs_fs_info *fs_info,
>  	return ret;
>  }
>  
> -int btrfs_dedupe_disable(struct btrfs_fs_info *fs_info)
> -{
> -	/* Place holder for bisect, will be implemented in later patches */
> -	return 0;
> -}
> -
>  static int inmem_insert_hash(struct rb_root *root,
>  			     struct inmem_hash *hash, int hash_len)
>  {
> @@ -317,3 +311,128 @@ int btrfs_dedupe_add(struct btrfs_fs_info *fs_info,
>  		return inmem_add(dedupe_info, hash);
>  	return -EINVAL;
>  }
> +
> +static struct inmem_hash *
> +inmem_search_bytenr(struct btrfs_dedupe_info *dedupe_info, u64 bytenr)
> +{
> +	struct rb_node **p = &dedupe_info->bytenr_root.rb_node;
> +	struct rb_node *parent = NULL;
> +	struct inmem_hash *entry = NULL;
> +
> +	while (*p) {
> +		parent = *p;
> +		entry = rb_entry(parent, struct inmem_hash, bytenr_node);
> +
> +		if (bytenr < entry->bytenr)
> +			p = &(*p)->rb_left;
> +		else if (bytenr > entry->bytenr)
> +			p = &(*p)->rb_right;
> +		else
> +			return entry;
> +	}
> +
> +	return NULL;
> +}
> +
> +/* Delete a hash from in-memory dedupe tree */
> +static int inmem_del(struct btrfs_dedupe_info *dedupe_info, u64 bytenr)
> +{
> +	struct inmem_hash *hash;
> +
> +	mutex_lock(&dedupe_info->lock);
> +	hash = inmem_search_bytenr(dedupe_info, bytenr);
> +	if (!hash) {
> +		mutex_unlock(&dedupe_info->lock);
> +		return 0;
> +	}
> +
> +	__inmem_del(dedupe_info, hash);
> +	mutex_unlock(&dedupe_info->lock);
> +	return 0;
> +}
> +
> +/* Remove a dedupe hash from dedupe tree */
> +int btrfs_dedupe_del(struct btrfs_fs_info *fs_info, u64 bytenr)
> +{
> +	struct btrfs_dedupe_info *dedupe_info = fs_info->dedupe_info;
> +
> +	if (!fs_info->dedupe_enabled)
> +		return 0;
> +
> +	if (WARN_ON(dedupe_info == NULL))
> +		return -EINVAL;
> +
> +	if (dedupe_info->backend == BTRFS_DEDUPE_BACKEND_INMEMORY)
> +		return inmem_del(dedupe_info, bytenr);
> +	return -EINVAL;
> +}
> +
> +static void inmem_destroy(struct btrfs_dedupe_info *dedupe_info)
> +{
> +	struct inmem_hash *entry, *tmp;
> +
> +	mutex_lock(&dedupe_info->lock);
> +	list_for_each_entry_safe(entry, tmp, &dedupe_info->lru_list, lru_list)
> +		__inmem_del(dedupe_info, entry);
> +	mutex_unlock(&dedupe_info->lock);
> +}
> +
> +/*
> + * Helper function to wait and block all incoming writers
> + *
> + * Use rw_sem introduced for freeze to wait/block writers.
> + * So during the block time, no new write will happen, so we can
> + * do something quite safe, espcially helpful for dedupe disable,
> + * as it affect buffered write.
> + */
> +static void block_all_writers(struct btrfs_fs_info *fs_info)
> +{
> +	struct super_block *sb = fs_info->sb;
> +
> +	percpu_down_write(sb->s_writers.rw_sem + SB_FREEZE_WRITE - 1);
> +	down_write(&sb->s_umount);
> +}
> +
> +static void unblock_all_writers(struct btrfs_fs_info *fs_info)
> +{
> +	struct super_block *sb = fs_info->sb;
> +
> +	up_write(&sb->s_umount);
> +	percpu_up_write(sb->s_writers.rw_sem + SB_FREEZE_WRITE - 1);
> +}

Please use the sb_ helpers, don't open code this.

> +
> +int btrfs_dedupe_disable(struct btrfs_fs_info *fs_info)
> +{
> +	struct btrfs_dedupe_info *dedupe_info;
> +	int ret;
> +
> +	dedupe_info = fs_info->dedupe_info;
> +
> +	if (!dedupe_info)
> +		return 0;
> +
> +	/* Don't allow disable status change in RO mount */
> +	if (fs_info->sb->s_flags & MS_RDONLY)
> +		return -EROFS;
> +
> +	/*
> +	 * Wait for all unfinished writers and block further writers.
> +	 * Then sync the whole fs so all current write will go through
> +	 * dedupe, and all later write won't go through dedupe.
> +	 */
> +	block_all_writers(fs_info);
> +	ret = sync_filesystem(fs_info->sb);
> +	fs_info->dedupe_enabled = 0;
> +	fs_info->dedupe_info = NULL;
> +	unblock_all_writers(fs_info);

This is awful, don't do this.  Thanks,

Josef